Time series analysis of gamma densitometry signals

Single, narrow-beam densitometry has been developed as a method for determining the flow regime and void fraction for industrial liquid gas experiments at high pressures and temperatures in a vertical, thick-walled, steel vessel. To develop suitable techniques, the experimental conditions were simulated using a transparant air/water column. In the transition region from bubbly to slug flow, a time sequence of four regimes, viz. annular flow, partially developed annular flow, coalescing bubble flow and bubbly flow were visually identified in a given cross section. Gamma rays were used to interrogate a column diameter, and digital time series analysis methods were applied. Amplitude spectral densities were used to determine any periodicity in the gas phase flow. The average void fraction for periodic gas flows was obtained by analysis of probability density distributions (PDD). The time sequence of the flow regimes was obtained from the signal magnitude of the diametral void fractions and the time spent in each regime was measured by the associated probability. The results compared well with those obtained from other methods. In the bubbly flow region, the standard deviation of the PDD exceeded that expected for nuclear counting. This bubble noise was assessed with respect to bubble properties.     

Two-phase bubbly flows in microgravity: Some open questions

Several studies on gas-liquid pipe flows in micro gravity have been performed. They were motivated by the technical problems arising in the design of the thermohydraulic loops for the space applications. Most of the studies were focused on the determination of the flow pattern, wall shear stress, heat transfer and phase fraction and provided many empirical correlations. Unfortunately some basic mechanism are not yet well understood in micro gravity. For example the transition from bubbly to slug flow is well predicted by a critical value of the void fraction depending on an Ohnesorge number, but the criteria of transition cannot take into account the pipe length and the bubble size at the pipe inlet. To improve this criteria, a physical model of bubble coalescence in turbulent flow is used to predict the bubble size evolution along the pipe in micro gravity, but it is still limited to bubble smaller than the pipe diameter and should be extended to larger bubbles to predict the transition to slug flow. Another example concerns the radial distribution of the bubbles in pipe flow, which control the wall heat and momentum transfers. This distribution is very sensitive to gravity. On earth it is mainly controlled by the action of the lift force due to the bubble drift velocity. In micro gravity in absence of bubble drift, the bubbles are dispersed by the turbulence of the liquid and the classical model fails in the prediction of the bubble distribution. The first results of experiments and numerical simulations on isolated bubbles in normal and micro gravity conditions are presented. They should allow in the future improving the modelling of the turbulent bubbly flow in micro gravity but also on earth.     

Two-phase bubbly flows in microgravity: Some open questions

Several studies on gas-liquid pipe flows in micro gravity have been performed. They were motivated by the technical problems arising in the design of the thermohydraulic loops for the space applications. Most of the studies were focused on the determination of the flow pattern, wall shear stress, heat transfer and phase fraction and provided many empirical correlations. Unfortunately some basic mechanism are not yet well understood in micro gravity. For example the transition from bubbly to slug flow is well predicted by a critical value of the void fraction depending on an Ohnesorge number, but the criteria of transition cannot take into account the pipe length and the bubble size at the pipe inlet. To improve this criteria, a physical model of bubble coalescence in turbulent flow is used to predict the bubble size evolution along the pipe in micro gravity, but it is still limited to bubble smaller than the pipe diameter and should be extended to larger bubbles to predict the transition to slug flow.     

Gas-liquid two phase flow pattern evolution characteristics based on detrended fluctuation analysis

In this paper, we first extract a nonlinear time series from the Weierestrass function as a toy model and investigate the anti-noise ability of six different fractal scale algorithm. The results indicate that the fractal scales calculated from Detrended Fluctuation Analysis(DFA) are robust with respect to variation in noise level. Based on the conductance fluctuating signals measured from vertical gas-liquid two phase flow experiment, we calculate the fractal scales of five typical flow patterns. The results show that when the water superficial velocity ranging from 0.0453ms⁻¹ to 0.226 ms⁻¹ and the gas superficial velocity ranging from 0.0043ms⁻¹ to 3.43 ms⁻¹, the values of the fractal scale of bubble flow are lowest corresponding to the random complex dynamic behavior, while the values of slug flow are highest corresponding to the alternatively periodic motions between gas slug and liquid slug, and the values of churn flow lies between them indicating the relatively complex dynamic behavior. Our main result is that the fractal scales obtained from conductance fluctuating signals can not only effectively characterize the dynamic characteristics of gas-liquid two phase flow patterns, but also further provide valuable reference for understanding the transitions of different gas-liquid two phase flow patterns.     

Frequency characteristics of the noise of R600a refrigerant flowing in a pipe with intermittent flow pattern

The acoustic characteristics of a long-shaped cylindrical bubble for slug or churn flow in a pipe are different from those of a freely rising spherical bubble in infinite liquid. In this research, the theoretical estimation of the natural frequency of the long-shaped cylindrical bubble was derived using the energy conservation law for a single bubble in a pipe. The acoustic characteristics of bubbles in a pipe were also investigated with the R600a refrigerant, which is widely used in refrigerators when the flow pattern in a pipe is slug or churn flow. In order to make slug and churn flow artificially, refrigerant-supplying equipment was designed and developed. Using this test equipment, the frequency characteristics of the long-shaped cylindrical bubble in 2-phase flow were investigated experimentally.     

R134a-DMF垂直管内鼓泡吸收特性研究

为研究垂直管内R134a-DMF(二甲基甲酰胺)鼓泡吸收过程的热、质传递特性,本文搭建了垂直管内鼓泡吸收实验测试装置,构建了管内R134a被R134a-DMF混合溶液鼓泡吸收过程的热、质传递数学模型.进一步通过模型分析了当吸收压力为0.35 MPa,蒸气入口温度为5℃,稀溶液入口质量流量为12.0 kg/h时,吸收过程...     

应用双头电导探针技术测量 气液两相泡状流局部参数

本文研究应用双头电导探针技术测量气泡局部参数,从而揭示了气液两相泡状流的内部流动规律。成功地设计了一种能够快速可靠测量气泡局部统计参数,包括空隙率、气泡速度、气泡尺寸、界面浓度等的电导探针系统。发现探针尖部的导通距离、沿流动方向两探针间的距离和两个探针针尖的间隙是设计电导探针的关键尺寸。     

Visualization of bubble behavior and bubble diameter correlation for NH3–H2O bubble absorption

The objectives of this paper are to visualize the bubble behavior for an ammonia–water absorption process, and to study the effect of key parameters on ammonia–water bubble absorption performance. The orifice diameter, orifice number, liquid concentration and vapor velocity are considered as the key parameters. The departing bubbles tend to be spherical for surface tension dominant flow, and the bubbles tend to be hemispherical for inertial force dominant flow. A transition vapor Reynolds number is observed at a balance condition of internal absorption potential (by the concentration difference) and external absorption potential (by the vapor inlet mass flow rate). As the liquid concentration increases, the transition Reynolds number and the initial bubble diameter increase. The initial bubble diameter increases with an increase of the orifice diameter while it is not significantly affected by the number of orifices. Residence time of bubbles increases with an increase in the initial bubble diameter and the liquid concentration. This study presents a correlation of initial bubble diameter with ±20% error band. The correlation can be used to calculate the interfacial area in the design of ammonia-water bubble absorber.     

应用双头电导探针技术测量气液两相泡状流局部参数

本研究应用双头电导探针技术测量气泡局部参数,从而揭示了气液两相泡状流的内部流动规律。     

液氢加注系统竖直管道内Taylor气泡的行为特性

针对液氢加注系统竖直管道内气液两相流实验化困难的问题,运用建模仿真的方法建立了竖直管道内Taylor气泡的运动模型,对Taylor气泡的形成过程、大小以及充分发展的Taylor气泡上升速度进行了研究.采用VOF方法对气液两相的交界面进行追踪,并引入CSF模型对两相间的表面张力进行计算.仿真结果表明:Taylor气泡是由...     

Effect of tube diameter on boiling heat transfer and flow characteristic of refrigerant R32 in horizontal small-diameter tubes

This study investigated the effect of tube diameter on flow boiling characteristics of refrigerant R32 in horizontal small-diameter tubes with 1.0, 2.2, and 3.5 mm inner diameters. The boiling heat transfer coefficient and pressure drop were measured at 15 °C saturation temperature. The effects of mass velocity, heat flux, quality, and tube diameter were clarified. The flow pattern of R32 for adiabatic two-phase flow in a horizontal glass tube with an inner diameter of 3.5 mm at saturation temperature of 15 °C was investigated. Flow patterns such as plug, wavy, churn, and annular flows were observed. The heat transfer mechanisms of forced convection and nucleate boiling were similar to those in conventional-diameter tubes. In addition, evaporation heat transfer through a thin liquid film in the plug flow region for low quality, mass velocity, and heat flux was observed. The heat transfer coefficient increased with decreasing tube diameter under the same experimental condition. The fictional pressure drop increased with increasing mass velocity and quality and decreasing tube diameter. The experimental values of the heat transfer coefficient and frictional pressure drop were compared with the values calculated by the empirical correlations in the open literature.     

Convective heat transfer coefficients for near-horizontal two-phase flow of nitrogen and hydrogen at low mass and heat flux

Correlations for convective heat transfer coefficients are reported for two-phase flow of nitrogen and hydrogen under low mass and heat flux conditions. The range of flowrates, heat flux and tube diameter are representative of thermodynamic vent systems (TVSs) planned for propellant tank pressure control in spacecraft operating over long durations in microgravity environments. Experiments were conducted in normal gravity with a 1.5° upflow configuration. The Nusselt number exhibits peak values near transition from laminar to turbulent flow based on the vapor Reynolds number. This transition closely coincides with a flow pattern transition from plug to slug flow. The Nusselt number was correlated using components of the Martinelli parameter and a liquid-only Froude number. Separate correlating equations were fitted to the laminar liquid/laminar vapor and laminar liquid/turbulent vapor flow data. The correlations give root-mean-squared (rms) prediction errors within 15%.     

气淮两相流流型BP网络识别

解决气液两相流流型的客观、自动识别一直是工业界和学术界所关注的问题。本以U型科垂直上升段内空气水两相流为例,采用压力、压差波动的不同特征参数的组合,深入研究了对流型自适应BP网络识别的准确率的影响,获得了能够满足工种精度要求的参数波动特征组合方式,本的结果表明,该方法需要建立大量的流型特征的数据库,适用于流型的离线识别。     

Bubbly Jet Impingement in Different Liquids

The impingement of bubbly jets in distilled water and ethanol has been experimentally studied on ground. An experimental apparatus for the study of jet impingement on ground and in microgravity has been designed. The opposed-jet configuration with changeable orientation is used in order to study which is the better disposition to achieve an efficient mixing process. The impact angle between jets that can be changed from 0° (frontal collision) up to 90° (perpendicular collision). The impinging jets are introduced into a test tank full of liquid by means of two bubble injectors. The bubble generation method, insensitive to gravity level for low Bond numbers, is based on the creation of a slug flow inside a T-junction of capillary tubes of 0.7 mm of diameter. Bubble velocities at the injector outlet and generation frequencies can be controlled by changing gas and liquid flow rates. Individual bubble properties and coalescence events, as well as the whole jet structure are analyzed from the images recorded by a high speed camera. Bubble velocities are compared with the velocity field of a single-phase jet. Rate of coalescence between bubbles is found higher in ethanol than in water, creating a higher dispersion in bubble sizes.     

Evaluation of the approach based on the concept of hyperbolicity breaking for prediction of flooding velocity of both room temperature and cryogenic fluids

The theoretical approach for the prediction of flooding velocity based on the concept of hyperbolicity breaking was evaluated in the counter-current two-phase flow. Detailed mathematical derivations of neutral stability condition together with the correlation of the void fraction are presented. The flooding velocity is obtained by assuming that the wavelength at flooding is proportional to the wavelength of the fastest-growing wave at Helmholtz instability. Some available experimental data for different fluid pair flow in inclined tubes is adopted for comparison with the theoretical calculations, which includes the data of water/air, aqueous oleic acid natrium solution/air, Aq. butanol 2%/air and kerosene/air in the published papers, as well as the liquid nitrogen/vapor nitrogen by the present authors. The comparison of flooding velocity proves that the approach can predict the flooding velocity with accepted accuracy for the water/air and liquid nitrogen/vapor nitrogen flow if the tube diameter is greater than 9 mm. While the diameter is smaller than 9 mm, regardless of the inclinations and the fluid pairs, the error becomes larger relative to the cases of diameter larger than 9 mm. The calculations for small diameter cases also fail to predict the critical liquid velocity at which the flooding velocity of gas reaches the maximum value, as revealed by the experiments. The reasons for the increased errors were qualitatively explained.     

Boiling flow through diverging microchannel

An experimental study of flow boiling through diverging microchannel has been carried out in this work, with the aim of understanding boiling in non-uniform cross-section microchannel. Diverging microchannel of 4° of divergence angle and 146 μm hydraulic diameter (calculated at mid-length) has been employed for the present study with deionised water as working fluid. Effect of mass flux (118–1182 kg/m²-s) and heat flux (1.6–19.2 W/cm²) on single and two-phase pressure drop and average heat transfer coefficient has been studied. Concurrently, flow visualization is carried out to document the various flow regimes and to correlate the pressure drop and average heat transfer coefficient to the underlying flow regime. Four flow regimes have been identified from the measurements: bubbly, slug, slug–annular and periodic dry-out/ rewetting. Variation of pressure drop with heat flux shows one maxima which corresponds to transition from bubbly to slug flow. It is shown that significantly large heat transfer coefficient (up to 107 kW/m²-K) can be attained for such systems, for small pressure drop penalty and with good flow stability.     

Gas–Liquid Interfacial Friction Factor for the Transition from Stratified to Slug Flow

Calculations based on the slug stability model and simplified stratified flow model provide predictions of the critical liquid height and the critical superficial velocities of a stratified flow for the transition to a slug flow in a horizontal pipe. Since slug flow derives from different interfacial waves patterns, previous interfacial waves model in stratified gas–liquid flows brings about the discrepancy between theoretical prediction and experimental data. A partial analysis for this behavior is given, which recognizes that the values of gas–liquid interfacial friction factor at the onset of slug flow have been underestimated, especially at high gas flows, and they should be obtained indirectly from other measured variables. Modified correlations for the interfacial friction factor are presented and better agreement between predicted and measured critical superficial velocities has been obtained.     

竖直管路内流动液氢条件中弹状氢气泡运动速度

低温加注系统中的弹状流可能引发各种非稳现象,造成低温液氢传输管路的破坏.针对垂直管路内气液两相流实验化困难的问题,运用建模仿真的方法建立了竖直管路中弹状流气泡的运动模型,模拟了弹状氢气泡在流动液氢中的上升过程,并对其运动速度进行了研究.仿真结果表明:弹状氢气泡在流动液氢中运动时,其速度模型中的速度系数与流体的速度有关,...     

Predictions of Phase Distribution in Liquid–Liquid Two-Component Flow

Ground-based liquid–liquid two-component flow can be used to study reduced-gravity gas-liquid two-phase flows provided that the two liquids are immiscible with similar densities. In this paper, we present a numerical study of phase distribution in liquid–liquid two-component flows using the Eulerian two-fluid model in FLUENT, together with a one-group interfacial area transport equation (IATE) that takes into account fluid particle interactions, such as coalescence and disintegration. This modeling approach is expected to dynamically capture changes in the interfacial structure. We apply the FLUENT-IATE model to a water-Therminol 59^® two-component vertical flow in a 25-mm inner diameter pipe, where the two liquids are immiscible with similar densities (3% difference at 20°C). This study covers bubbly (drop) flow and bubbly-to-slug flow transition regimes with area-averaged void (drop) fractions from 3 to 30%. Comparisons of the numerical results with the experimental data indicate that for bubbly flows, the predictions of the lateral phase distributions using the FLUENT-IATE model are generally more accurate than those using the model without the IATE. In addition, we demonstrate that the coalescence of fluid particles is dominated by wake entrainment and enhanced by increasing either the continuous or dispersed phase velocity. However, the predictions show disagreement with experimental data in some flow conditions for larger void fraction conditions, which fall into the bubbly-to-slug flow transition regime. We conjecture that additional fluid particle interaction mechanisms due to the change of flow regimes are possibly involved.     

Void Fraction Prediction in Annular Two-Phase Flow Using an Algebraic Turbulence Model

The study considers the prediction of the main flow parameters of adiabatic annular two-phase flows, focusing in particular on the void fraction. Recently developed models to predict the entrained liquid fraction, the wall shear stress and the velocity profile in the annular liquid film are discussed and integrated to provide a unified annular flow model that is used to predict the void fraction. A comparison with experimental void fraction data shows that the unified annular flow model slightly outperforms some of the most accurate void fraction empirical correlations currently available.